1. Field of the Invention
The present invention relates to the channel estimation of a Multiple Input Multiple Output (MIMO) wireless communication system; and, more particularly, to a preamble structure designed to be robust against a timing offset in the channel estimation of an MIMO wireless communication system, and a channel estimation algorithm thereof.
2. Description of Related Art
Many researches have been made for overcoming the limitations of International Mobile Telecommunications 2000 (IMT-2000) and for establishing the fourth generation mobile communication network. The fourth generation mobile communication network is a heterogeneous communication network, not a homogenous communication network. For example, the fourth generation mobile communication network integrally includes various types of communication networks for satellite communication, wireless Local Area Network (LAN), and digital broadcasting.
International Telecommunication Union (ITU) defines that the fourth generation wireless communication supports a transmission rate of approximately 110 Mpbs while moving and a transmission rate of 1 Gpbs while not moving.
Recently, IEEE 802.11b, IEEE 802.11a, IEEE 802.11g, and IEEE 802.11n have been used as a wireless LAN specification. Particularly, a wireless LAN employing IEEE 802.11b or IEEE 802.11g has been popular. IEEE 802.11g supports up to a transmission rate of approximately 54 Mbps in a bandwidth of 2.4 GHz. IEEE 802.11n is currently in development and it is expected that IEEE 802.11n will support up to a transmission rate of approximately 540 Mpbs although it could not support a transmission rate of 1 Gbps which is required in the fourth generation wireless communication. In order to realize such high speed wireless communication, core technologies, Orthogonal Frequency Division Multiplexing (OFDM) and Multiple-Input Multiple-Output (MIMO), were introduced.
NTT DoCoMo Inc. predicted that Orthogonal Frequency Code Division Multiplexing (OFCDM) and OFDM will be used as a core technology for the fourth generation wireless communication. The OFCMD and OFDM have better performance than Direct Sequence-Code Division Multiple Access (DS-CDMA) based wireless communication access which has been used for a communication system using a wideband channel. In case of a wideband channel, it is possible to significantly reduce performance deterioration caused by the interference of multipath in OFCDM or OFDM.
Meanwhile, it is required to improve the spectral efficiency in order to increase a transmission rate with a frequency resource limited. As one of methods for improving the spectral efficiency, an MIMO technology has been used. Also Bell Laboratories Layered Space Time (BLAST) was used to use the MIMO technology. It has been widely known that the BLAST has better performance than that of Maximum Likelihood Detection (MLD) and Minimum Mean Squared Error (MMSE) in view of Bit Error Rate (BER) or Block Error Rate (BLER). However, it is not suitable to embody the MLD in a communication system because the BLAST has high computational complexity that exponentially increases according to a modulation scheme or the number of antennas. In order to reduce the high computational complexity of the MLD, MLD with OR decomposition and M-algorithm (ORM-MLD) was introduced. The ORM-MLD is MLD using the M-algorithm and QR decomposition. Although the ORM-MLD has the lower computational complexity than that of the MLD, there is a margin to improve the ORM-MLD.
As described above, the MIMO technology and multiple frequency bands are essentially necessary for high speed wireless communication that supports a transmission rate faster than that of the fourth generation wireless communication. Accordingly, it is necessary to develop an effective channel estimation method suitable to a wireless communication system using MIMO and multiple frequency bands. Also, a timing offset may significantly influence the performance of an MIMO wireless communication system. In this case, channel estimation may deteriorate the performance thereof. Therefore, it is also required to develop a channel estimation algorithm for improving the performance deterioration problem of the channel estimation.